Just for fun, I like to test sighting disances in a ballistics soft-ware package like 'Point-Blank'. I choose a mid-range trajectory height about equal to the height of sights above bore centreline, and see what zero is required to achieve it. Usually for the big-bores and open sights, using a 3/4 to 1" mid-range height, zero is at 60 to 70 yds. For a scoped rifle with 1 1/2" midrange height, it might be out towards 100 yds or just past.

In practice, though, the bench to target-frame distance at the local range has the greatest influence on sight zero for most of us!